The overall objective of this investigation is to investigate, at the level of the cell and cell membrane, the mechanisms by which the small intestine transports ions and water-soluble non-electrolytes (where the transport of these is coupled to that of ions, e.g. Na+), the factors that modify or regulate these transport processes, and the role of specific membrane transport processes in intestinal absorption and secretion. Because of their stability in vitro and their functional resemblance to their mammalian analogs, isolated Necturus intestine and gallbladder are used. Double barreled (open tip and ion-selective) microelectrodes are used to measure cell membrane potentials and intracellular ionic activities in intact tissue sheets. Single-channel recording is used to study ionic channels in lipid bilayer systems containing purified intestinal brush border (BBM) or basolateral membrane (BLM) fractions. Patch clamping with isolated intestinal absorptive cells will also be attempted. Major short-term objectives are, (i) to investigate the specific mechanisms (Na/Cl symport; parallel Na/H, C1/HCO3 antiport; K-dependent symport) involved in apical NaC1 entry and the factors that regulate them, (ii) to define the regulatory roles of intracellular Ca and cAMP in intestinal secretion, (iii) to identify and characterize ionic channels in intestinal BBM and BLM, (iv) to study the mechanisms that regulate ionic permeabilities in the intestine, and (v) to study Na-coupled sugar and amino acid transport in reconstituted bilayer systems. The long-term goal of these studies is to provide experimental information on and generate models of absorptive and secretory function that will assist the development and application of new regiments for the management and control of human gastrointestinal disorders.